When employing composite materials, for example, a bituminous mixture such as asphalt concrete, it is generally desirable to test the composition of the materials before installation to ensure that the installed material has desired properties of structural strength, durability and the like. For example, the "hot-mix" asphalt concrete used to pave roads, airport runways and the like desirably has a predetermined proportion of asphalt binder to aggregate and a predetermined gradation of aggregate size to help ensure that the material will have adequate and uniform application and wear properties. Accordingly, in the paving industry, determination of asphalt content and aggregate gradation in batches of asphalt concrete typically are critical quality assurance procedures.
Solvent extraction techniques have been widely used to determine asphalt content and aggregate gradation in asphalt concrete. According to these techniques, a sample of asphalt is weighed and then washed using a suitable solvent to remove the asphalt binder in the sample and leave a clean aggregate residue. The residue may then be weighed and compared to the prewashed weight of the sample to determine the asphalt content of the sample. The clean aggregate may also be sieved using a series of predetermined sieves to determine aggregate gradation.
Although solvent extraction techniques can effectively be used to determine asphalt content, they can have serious shortcomings. The solvent washing process typically is slow, a characteristic which undermines the utility of solvent extraction techniques in mass production environments in which rapid testing is desirable to ensure continuous quality control. The solvent washing process also tends to generate hazardous effluents which may pose a disposal problem, and traditionally employs chlorinated solvents traditionally which have been categorized as hazardous materials and accordingly have been banned in some governmental testing facilities. Alternative biodegradable solvents such as terpenes may be used, but tend to work in an even slower fashion than the chlorinated solvents they replace. Another alternative, the nuclear asphalt testing gauge, is an effective tool for measuring asphalt content but typically cannot perform aggregate gradation testing.
Promising alternative techniques which provide for both content and gradation analysis are pyrolysis techniques in which the asphalt binder in a sample of asphalt is burned off to leave an aggregate residue. Pyrolysis techniques are generally described in "Historical Development of Asphalt Content Determination by the Ignition Method," by Brown et al., and in "Solvent-Free, Nuclear-Free Determination of Asphalt Content and Gradation of Hot-Mix Asphalt Concrete, " by Todres et al., ASTM Journal of Testing and Evaluation, November 1994, 564-570. According to these techniques, a sample of asphalt concrete is placed in an oven or similar apparatus and heated to volatilize and combust the asphalt binder, thus separating the binder from the sample and leaving an aggregate residue. The temperature and other conditions at which pyrolysis occurs tend to be factors which strongly influence the accuracy of tests, as insufficient temperatures may not completely separate the binder and excessive temperatures can lead to aggregate loss and gradation changes induced by chemical changes in the aggregate and thermal shock. Several furnace-type apparatus have been developed for performing asphalt pyrolysis, including furnaces which incorporate an integral weighing scale in order to allow measurement of a sample of asphalt concrete during pyrolysis as described in, for example, U.S. Pat. No. 5,081,046 to Schneider et. al.
The asphalt binder in asphalt concrete typically includes a significant proportion of low-end hydrocarbons and impurities which are difficult to completely combust. Thus, one of the most vexing problems associated with asphalt pyrolysis is dealing with the volume of noxious, high particulate content smoke typically generated by the combustion of the asphalt binder. Although this noxious smoke may be exhausted out of the testing furnace using a fan or similar device, the smoke generated by heating of an asphalt sample generally is too noxious to directly exhaust into a laboratory exhaust system or similar environment, as direct discharge may produce an unacceptable level of pollution and may foul the exhaust system of the site in which the furnace is installed.
Several techniques for dealing with the smoke problem have been proposed. For example, an analyzing furnace previously developed by Troxler Electronic Laboratories, Inc. of Research Triangle Park, N.C., assignee of the present application, includes an afterburning chamber which receives and treats smoke and other byproducts of asphalt pyrolysis produced within a main furnace chamber connected thereto, exhausting cleaner gases from the afterburning chamber into a plenum and out of the furnace via a blower mounted on the plenum. Another furnace design employs filters designed to filter combustion products created by combustion of an asphalt sample within a combustion chamber of a furnace, as described in U.S. Pat. No. 5,558,029 to Peake.
Although these approaches may help combust smoke generated by pyrolysis of an asphalt sample, they may not provide optimal combustion conditions and thus, may render inaccurate and nonuniform results. Variations in exhaust characteristics at installation sites may lead to variation in combustion conditions. For example, a specimen of hot-mix asphalt may be divided into several samples which may be processed in different furnaces, even different furnaces at different testing sites. Variable combustion conditions in any of the furnaces may lead to inaccurate results, and nonuniform combustion conditions may result in nonuniform results among the furnaces. Moreover, nonoptimal combustion may lead to deleterious side effects such as poor emissions quality, formation of soot deposits in the furnace and exhaust system, and gaseous discharges into the testing site which may be harmful to personnel and equipment. Afterburners and filters may trap or burn some pollutants which otherwise might be discharged, but still may not produce the combustion and exhaust characteristics needed to reduce pollution and unwanted backflow emissions to an acceptable level.